A dreadful accident of the above description occurred on the 7th inst.,at Acton, Mass., by which several workmen were blown to pieces. The locality is known as Pratt's Powder Mills, and the manufacture is carried on in several detached buildings situated at a small distance from each other. Some workmen in the Kernel-housemdash;the filth building from themain road and from the dwelling housemdash;were employed in work upon a cylinder, when, doubtless, (for no one is left to tell the tale), a spark from a chisel falling upon the combustible material, an awful explosion took place, instantly killing the men, whose names were Hudson, Bal-combe, and Hanscom, two of whom were married and leave tamilies. The Kernel-house is usually considered the most dangerous of the several houses, and it contained, at this time, as commonly, a considerable amount ot. powder. The occurrence of this tragedy was, of course, instantaneous, and its consequences quite inconceivable to those who have never witnessed a scene of this description. Next the kernel-house was on the west side a mixing house, and this quite near; the explosive force forming a vacuum, the sides of the mixing house from the pressure of the air inside were instantly driven out, and the atmosphere, full of fire and cinders entering at an interval of some three seconds, as it is said, this house also exploded. The press-house (as it is called) is placed further ofi from the kernel-house than the last named, toward the east, and this distance doubtless saved the powder in it from exploding, though the boarding is mostly forced from its sides, and it is in fact a nearly complete wreck. Of the kernel-house it may be said, it is swept absolutely clean off from the very foundation ; the water dam, timbers, and all the machinery scattered to the four quarters of Heaven. The meadow adjoining has the appearance of being thickly sown over with fragments of boards and pieces of timber, while large portions of the roof lay in masses. On the top of the range of hills several large pieces of timber were carried, and countless smaller bits. The mixing house is not so utterly and entirely dispersed, but its timber and machinery (or what is left of it) lies a heap of smoking and blackened ruins. All the remains of these are as completely blackened over by the powder (not charred) as if the process had been effected by the brush of some diligent painter. Words cannot convey a faint impression of the scene of this fearful tragedy. In the workmen's dwellings near the mills the force of the concussion was very great. The window-glass was broken, the furniture shaken, and persons who were near thrown over. Some part ot a chimney at the distance of half a mile was thrown off, and in fact at several miles distance the shock was very perceptible.